1. Field of the Invention
The present invention relates to current amplifier circuits, and in particular to a programmable multi-gain current amplifier.
2. Description of the Related Art
Filters utilizing current amplifiers (CAs), unlike the second generation current conveyer (CCII) and its extended versions are pure current-mode circuits wherein the primary signal variable is current. Basically, a CA is equivalent to a current controlled current source with gain α and ideally zero input impedance and infinite output impedance. It is often defined as a two terminal device that amplifies a current input signal applied at its low impedance input terminal X and conveys it to a high impedance output terminal Z. A current follower (CF) is a unity gain CA. From a certain point of view, it is the simplest current-mode device since it is a subpart of several devices such as CCII, current feedback amplifier (CFA) and transresistance amplifier (TRA).
In the area of filter design, CA based topologies avoid the use of any explicit or implicit voltage-mode active component leading to true current-mode signal processing circuits having the potential to operate at relatively high frequency and large signal swings. In fact, the bandwidth and the closed-loop gain are almost independent and a high voltage swing is not usually required. Known configurations include a current-mode Sallen-Key low-pass filter based on the CA and other filters based on CFs.
A current controlled current conveyor is widely used to provide the missing tuning feature of its CCII based counterparts. A programmable CA would be obtained from a CCCII by grounding its Y terminal. A programmable integrator can be obtained from a single output CCCII but this would require applying an input voltage (instead of the usual current signal) at the X-terminal of the CCCII. On the other hand, a current-mode programmable integrator can be obtained by using a dual-output CCCII adopting the topology described previously for the CA, or it can be obtained by using two CCCII.
Alternatively, electronically controllable CCIIs (ECCIIs) are obtained with the help of small-signal current amplifiers as suggested in the prior art. Basically, the sensed output current (ix) of the voltage buffer stage is applied to the input of a current amplifier. The current amplifier amplifies ix and makes it available from a high output impedance terminal Z. The current gain of these current amplifiers is often a function of ratio between two biasing currents. Thus, the gain is controlled by varying these biasing currents. A BJT based ECCII is known in the art and complementary metal oxide semiconductor (CMOS) based ECCIIs are also known. In the known CMOS circuits, however, the maximum gain is limited by the size of some current mirror transistors (e.g. with equal size current mirrors the gain is limited to 2). Large transistors require large silicon area and result in limited bandwidth as they would be associated with larger practice capacitances. The known programmable CCII replaces the current amplifier with several cascaded current division cells. Each cell consists of seven transistors with the first two cells (among n cells) consuming 300 μA leading to an area and a power inefficient solution.
A CCII with both voltage and current gain called VCG-CCII is also known in the art. The voltage gain Vx/Vy, just like any voltage amplifier, suffers from gain-bandwidth product problems. Whereas the current gain Iz/Ix is proportional to the small signal transconductance or output resistance of current mirroring transistors. The bandwidths of the current transfer characteristics of the VCG-CCII of these known works are limited to approximately 20 kHz and 1 MHz, respectively. In addition, the operation of these circuits is valid only for small signal processing, limiting their linearity.
A highly linear programmable CCII based on a programmable current mirror of [25] is also known in the art. However, this technique employs transistors operating in moderate inversion which results in low bandwidth (17 MHz) and limited tuning range. Adjustable current mirrors such as the known cell in the art can also be utilized but extending the design to produce multi-output would be associated with increased circuit complexity. Additional known art presents a single output programmable current amplifier employing two SINH or two TANH blocks, for example. But this known topology is clearly inefficient, since each of the two blocks consists of a CCII and a CM with adjustable gains. Another known technique is the design of a digitally controlled fully differential current conveyor (DCFDCCII). It utilizes two current division networks (CDN) in order to realize a single DCFDCCII.
In known current mirror CM based circuits, the input stage transconductance (basically 1/gm) is used for electronic programmability. Hence, it is possible to realize active-C filters based on CMs under the condition that each CM in the circuit topology is loaded by series resistance at the X terminal. In this case, the passive resistors would be replaced by the internal input resistance of the CM. An adjustable biasing current is used to vary the parasitic resistance (1/gm) of the CM's X-terminal. However, this approach cannot be employed to provide independent tuning characteristics since it is impossible to obtain different gains from the same device. This leads to filters exhibiting dependent pole frequencies and pole quality factors.
The second unattractive feature of these known filters is that various gains are often fixed to unity. A third problem with this known approach is that 1/gm is naturally nonlinear which significantly limits the linearity and the tuning range. Also, it can be seen that filters based on CM devices inherently cannot provide virtual ground input impedance (i.e., 1/gm is not ideally zero). Yet there remains the problem of gain programmability and a number of independently programmable outputs.
Thus, a programmable multi-gain current amplifier addressing the aforementioned problems is desired.